N-Substituted lactams can be prepared, according to a standard process, by reaction of the alkali metal salts of lactams with alkylating agents, for example as described in U.S. Pat. No. 4,122,170. The alkali metal salts used as starting compounds have to be prepared beforehand by reaction of alkali metals or alkali metal hydrides with the corresponding lactams, in the presence of an inert solvent and under a nitrogen atmosphere. This process is not economical due to the high cost of alkali metals or alkali metal hydrides and the large amounts of solvent which are required.
Another process has been disclosed by G. L. Isele et al. (Synthesis, 1971, 266), which consists of alkylating caprolactam with an alkyl bromide or chloride in the presence of dimethyl sulphoxide as solvent and of potassium hydroxide as acceptor of hydrohalic acid. The advantageous results of this process would be due to the action of the solvent, which would promote the formation of the potassium salt of the lactam. However, this process is not economical on an industrial scale. Potassium hydroxide is relatively expensive. The not very common solvent is expensive and used in large amounts. The separation of the product obtained and the dimethyl sulphoxide can, moreover, present problems when the boiling points of these compounds are similar.
According to the process described in U.S. Pat. No. 3,865,814, lactams are reacted with an alkyl or aralkyl halide in the presence of an alkali metal hydroxide and in the absence of solvent but a solvent is nevertheless used in all the examples in order to recover the portion of N-substituted lactams contained in the salt formed, which salt is separated by filtration. Moreover, due to the high exothermicity of the reaction, it is indicated that it is preferable, in carrying out the process, to add the alkali metal hydroxide to the mixture of lactam and halogenated organic compound and/or to add an inert solvent with an appropriate boiling point. Large amounts of by-products, such as ethers, are formed. Fractional distillations are then required in order to recover the N-substituted lactams and the latter are still contaminated by a significant amount of unreacted lactams. The yields are low. This process is consequently not suited to industry either. The N-substituted lactams are insufficiently pure and the heat given off by the reaction is difficult to control on an industrial scale.
More recently, processes for the preparation of 1-dodecylazacycloheptan-2-one are described, in U.S. Pat. No. 4,812,566 and U.S. Pat. No. 4,973,688, in which a phase transfer catalyst and organic hydrocarbon solvents, such as benzene, toluene, xylene, cyclohexane, n-hexane and petroleum ether, are used. It is necessary to use amounts of solvents such as from 1 to 3 times the total weight of the reactants. These solvents are inflammable and, more- over, some are toxic, such as benzene and toluene. The storage, the recycling and the removal of these solvents require many operations in order to have regard for safety and the environment, which significantly increases the cost of the process.